1. Field of the Invention
The present invention relates to techniques for phasing the elements of a phased array antenna to provide wide area coverage in a failure mode and, more particularly, to techniques for locking each of the plurality of phase shifters of a phased array antenna to a separate predetermined phase shift value in a manner whereby the combined radiations from all array elements produce a curved wavefront such as, for example, a spherical wavefront which provides an area coverage beam rather than the usual flat, or planar, wavefront.
2. Description of the Prior Art
Phased array antennas are used for many applications and have been proposed for use in satellites for directing beams to various earth stations in a communication system. For example, in the article "A Scanning Spot-Beam Satellite System" by D. O. Reudink et al in The Bell System Technical Journal, Vol. 56, No. 8, October 1977 at pages 1549-1560 the use of a phased array antenna for providing a scanning spot beam which is scanned over a service region on the earth was proposed. There, the advantage of the scanning beam over an area coverage beam was stated as the use of less RF power and a concomitant reduction in required electrical power generation equipment weight, and of experiencing a gradual decrease in power as individual elements of the antenna system fail; i.e., it is a system which fails "gracefully" because of the built in redundancy of the many elements in the phased array. However, the failure of an electrical component which provides a signal to the phased array antenna could stop the signal flow altogether until repaired or replaced.
U.S. Pat. No. 3,119,965, issued to E. N. Phillips on Jan. 28, 1964, relates to an arrangement using an active power splitter for applying an input signal in parallel paths to an antenna array. The patent states that, in the disclosed arrangement, an output signal from a frequency generator is applied to an active ultra-high-frequency power dividing device having a plurality of output ports. Each output port of the power splitter is connected to one element of an antenna array which provides a directional ultra-high-frequency radiation. The active power splitter can provide amplification of the ultra-high-frequency signal and additional amplification can be provided for each of the parallel signals between the output ports and their respective antenna elements. This additional amplification can be selectively controlled externally, either to maintain the amplification of all the parallel output signals constant or to vary the amplification of the output signals relative to each other. In addition, the relative phases of the parallel output signals can be controlled selectively externally either to maintain all output signals in phase or to change the relative phases of the output signals in order to alter the directivity of the antenna array.
One unit, however, contained in, for example, a scanning spot beam satellite system for which it is difficult to provide redundancy is the on-board computer which controls the phase shifters of the phased array antenna because of its complexity of operation and size. The problem remaining in the prior art, therefore, is to provide a technique which allows continued operation of the phased array antenna system upon the failure of a phase shifter controller for providing signal transmissions to each of the spaced-apart remote receivers.